Two-anode discharge detector for microwaves



Dec. 13, 1960 J. M. ANDERSON TWO-ANODE DISCHARGE DETECTOR FOR MICROWAVES Filed Jan. 2, 1959 Fig, 4.

M 7 W m 4am 60 2 5 4 Unit 1 7' TWO-ANODE DISCHARGE DETECTOR FOR MICROWAVES Filed Jan. 2, 1959, Ser. No. 784,713

8 Claims. (Cl. 315-39) This invention relates to a gaseous discharge apparatus and more particularly to a detector for indicating the presence and intensity of modulation of a microwave.

It is known that a microwave impressed upon various regions of a gaseous discharge is effective to impart energy to the free electrons in their regions, the effect of which is to alter the conductive properties of the discharge. A microwave impressed upon the plasma of the positive column of a discharge is effective to increase the ionization rate therein to decrease the potential field in this region and thus to increase the current conduction through the discharge. A microwave impressed upon the Faraday Dark Space region of a gaseous discharge having low energy electrons of high density is effective predominantly to cause diffusion of the high density of electrons in this region away therefrom by reason of the energy absorbed by the electrons from the field. An increase in electric field gradient follows with a consequent decrease in current. The interaction of the microwave with the negative glow results in substantially the same current change as noted for the positive column. Thus, an observation in the change in intensity and current in the external circuit producing such a gaseous discharge enables a determination of the presence or absence of the microwave field or the magnitude thereof since these effects vary with the presence and intensity of the microwave field. Devices employing the technique described above are known but the same have characteristics limiting their utility as detectors. Presently such devices are responsive to a minimum detectable signal of 20 db higher than that for a crystal detector and video frequency responses thereof are generally limited to approximately 50 kc. per second.

Accordingly, it is a primary object of my invention to facilitate microwave detection in a gaseous discharge device without the aforementioned limitation.

It is another object of my invention to achieve microwave detection with a minimum of noise produced by random variations in space potential.

In accordance with features of my invention the foregoing objects are achieved by observing the difference in current established in electrodes projected at spaced points in a gaseous discharge plasma upon which an electromagnetic wave is impressed. Microwave detectors utilizing the microwave interaction occurring at the very surface of only a single probe are disclosed and claimed in my copending application for Gaseous Discharge Structures, Serial No. 784,746, filed concurrently herewith. One of the electrodes is positioned at a location of high interaction between the field and the plasma and the other electrode is positioned at a location of relatively low interaction. Thus the conduction in the separate electrodes will be different according to the interaction in the region of the electrode whereby a potential difference may be established therebetween and across resistors in the output circuit thereof and which varies with the differential in conduction.

ttes Patent Other and further objects and advantages will appear from a consideration of the following detailed description of the invention taken with the accompanying drawings in which:

Fig. 1 is a schematic representation of a dual anode gaseous discharge tube useful in explaining the operation of my invention;

Fig. 2 is a graph representing the current values as the ordinate at respective anodes in the tube of Fig. 1 with the difference in applied potentials being plotted as the abscissa;

Fig. 3 represents an arrangement of tube elements in a wave guide effecting an interaction between electromagnetic waves and an ionized gas in the wave guide;

Fig. 4 is a partially cut away elevational view of a wave guide structure adapted for operation as a detector;

Fig. 5 is a detail view in section of the wave guide structure shown in Fig. 4 and taken along section 5-5 therein;

Fig. 6 is a schematic representation of a gaseous discharge tube structure having means for introducing an electromagnetic wave for interaction with an ionized gas;

Fig. 6a is a sectional end view taken along lines 6a-6a of the tube shown in Fig. 6; and

Fig. 7 is a cross-sectional view of a wave guide and gaseous discharge tube according to another embodiment of my invention.

Referring now more particularly to Fig. 1 of the drawings, 10 represents generally and schematically a gaseous discharge detector according to my invention in its entirety and includes an envelope 11 which may be glass or other suitable material transparent to microwaves, filled with an ionizable gas such as one of the noble gases at a pressure of the order of 1 mm. of mercury. A relatively large cathode electrode 12 is disposed in envelope 11 at one end thereof and a pair of anode electrodes 14 and 16, relatively small with respect to the cathode, are disposed side by side at the other end of the glass envelope. Cathode 12 which may be coated with a suitable electron emission enhancing material such as barium, is made negative with respect to ground as a reference by a direct potential source represented at 18 connected between ground and cathode 12 through variable resistor 20. Anode 14 is connected to ground through a resistor 22 and anode 16 is connected to ground through a resistor 24. The size and spacing between the tube electrodes and the pressure of the ionizable gas in the envelope 12 are selected and correlated so as to produce a gaseous discharge in a negative glow condition upon the application of the potential 18. Purely as an example, the anodes may be spaced inch apart, the anode cathode spacing may be .400 inch and the interelectrode region may be filled with neon at a pressure of substantially one mm. of mercury. Also, resistors 22 and 24 are selected to be of equal value and the positioning of anodes 14 and 16 are such as to establish a balanced circuit between ca-hode 12 and each of the anode electrode circuits. Upon the application of potential 18 to the circuit as shown in Fig. l, a negative glow type of gaseous discharge is produced in the tube and under the circumstances where the circuit is not subject to any other influence, a current will flow from the cathode 12 to each of the anodes 14 and 16. Such currents are graphically represented in Fig. 2 where it is observed that i the current collected at anode 14, equals i the current collected at anode 16, when no microwave energy is impressed on the gaseous discharge and potentials V and V appearing across respective resistors 22 and 24 are of equal value to produce a zero diiferential potential, AV. In the interelectrode regions adjacent to each of the anodes the space potentials will be substantially equal and the electron density and electron temperatures will also be substantially equal.

Upon the incidence of a microwave on the discharge of one of these regions, the free electrons in the region .absorb energy from the microwave. The electron temperature is increased and the electrodes are dilfusedaway from such a region to other regions within the envelope 12 including the region adjacent to the other anode and thus a contact potential is established between the plasmas adjacent to the anodes where difierences in electron temperature exist. The potential difference between the aifected plasma region and the adjacent anode increases and the discharge current to this anode decreases. The total discharge current from the cathode will tendto remain constant which implies an increase of current to the other anode. Accordingly, the potential differential across theanodes isestablished as designated by AV in Fig. 1 since the IR drops across resistors 22 and 24dilfer. Such a potential differential varies as' the intensity of the electromagnetic field and thus gives an indication of the magnitude of the same or in other words it detects the incoming signal.

As shown in Fig. 2 of the drawings, the substantially constant value of cathode current is apportioned between anodes 14 and 16 from a very low value to either one of the anodes to substantially the entire cathode current to either anode, depending upon the microwave intensity. According to a feature of my invention, the differential potential AV generated across the anodes 14 and 16 is largely free of variations produced by the random variations in space potential of the negative glow plasma since the same eifects are produced in equal magnitude at the respective anodes and the same are oppositely directed in the external circuit to cancel each other out. Only the potential differential created by the effect of the microwave appears in the output taken across the anodes.

A practical application of my invention to a wave guide system is shown in Fig. 3 of the drawings in which a wave guide represented at 26 and having a short circuiting plunger 28 at one end thereof, is provided with a window 30 of material such as mica which is transparent to electromagnetic wave energy but impervious to gas, to seal off a region 32 within the wave guide for containing an ionizable gas at low pressure. Within the region 32 is disposed a cathode 34 which may be made of the metal such as molybdenum and may be coated with an electron emission enhancing material such as barium, extending along a length of the upper wall of the wave guide 26, and facing the cathode from the opposite side of the wave guide are disposed a pair of anodes '36 and 38 spaced a quarter wavelength apart at a predetermined frequency of signals to be detected due consideration being given to the phase shift introduced by the negative glow region of the ionized gas as fully disclosed and claimed in my copending application for Electrode Structure for Microwave Electronic Phase Shifters and Attenuators Using Gaseous Discharges, Serial No. 784,686, filed concurrently herewith. The cathode and each of the anodes are insulated from respective walls of the wave guide and each is provided with a lead extending through the corresponding wave guide wall and being insulated therefrom by respective insulators 40, 42 and 44 disposed in tubular supports 46, 48 and 49 leading to apertures in the wave guide. The end of the wave guide is sealed from ambient space and in accordance with Well known principles, the short circuiting plunger 28 is movable along the wave guide to establish a variable termination of the wave guide for positioning standing waves created therewithin. In many cases plunger 28 may be replaced by a solid metal wall termination for the sake of simplicity. For ionizing the gas in region 32 a positive direct potential from source 50 is applied to each of the anodes 36 and 38 through respective resistors 52 and 54 and the negative terminal of source 50 is applied through the resistor 53 to cathode 34. The magnitude of source 50 is sufficiently great to establish an ionizing potential gradient between cathode 34 and each of the anodes 36 and 38 whereby a gaseous discharge in a negative glow condition is established in the regions near the anodes within the tube.

In the operation of the detector shown in Fig. 3, electromagnetic wave energy of the predetermined frequency mentioned above is propagated from a suitable source through a wave guide 51 connected to wave guide 26 and then along wave guide 26. Short circuiting plunger 28 is positioned at a location near anode 38 whereby a minimum intensity of the standing microwave established within the tube appears in the vicinity of this anode and the maximum intensity of standing microwave is established in the vicinity of anode 36. Thus, a minimum interaction between the electromagnetic wave propagated within the wave guide occurs in the region of the anode 38 while a maximum interaction occurs in the region of the anode 36. Under these circumstances the conduction in anode 36 and the external circuit thereof is considerably less than the conduction in the anode 38 and its circuit whereby a differential potential AV is established between these anodes indicative of-the magnitude of the standing wave established within the waveguide 26.

For avoiding any reflections or other disturbances caused by the cathode within the interaction region a further and more advantageous manifestation of the embodiment of the invention shown in Fig. 3 may be as shown in Figs. 4 and 5 of the drawings wherein a wave guide 26' is provided with a recessed portion 56 established in onewall thereof by end tapered inserts 5'5 and 57 and along which recess a cathode 34' is mounted. The width of recess 56 is shown exaggerated but is so small that only a fringing of electromagnetic wave energy propagated along the wave guide 26 will appear in the recess portion 56. Anodes 36' and 38 are probes extending into the wave guide from one side wall thereof and are spaced substantially one quarter wavelength apart at the frequency of energy propagated within the wave guide and to be detected. Each probe is insulated and supported from the wave guide as shown at 42 in Fig. 5 of the drawings. Cathode electrode 34' is similarly insulated and spaced from the walls of recess 56 and is provided with an exterior terminal connection 60 extending through insulator 40' in sleeve 46 communicating with the interior of the recess 56. A short circuit plunger such as 28 of Fig. 3 is necessary with the embodiment of Fig. 4 in order to establish a standing microwave in the regionof the probes 36 and 38'. The plunger may be between windows 30' and 31 or external thereto in'an adjacent Wave guide region.

In accordance with another embodiment of my invention as shown in Fig. 6 of the drawing, the essential elements of a microwave detector are contained in an envelope 68 which may be glass or any other suitable microwave transparent material. These elements include a hollow tubular cathode 70 of circular crosssectionor of oval cross-section as shown in more detail in Fig. 6a of the drawings, and extending longitudinally along the interior of the cathode are a helically wound anode electrode 72 and a rod anode electrode 74 substantially equally spaced from the cathode and from each other along their lengths. For steady state balance the diameters of the rod material from which the anodes are formed are proportioned so that the total exposed surface area of the longer helical anode 72 is substantially the same as the exposed surface area of anode 74. The anodes terminate in exterior terminals 76 and 78 for exterior connections thereto and the cathode 70 is connected to an external terminal 80 for purposes of potential application.

Electromagnetic wave energy is applied to the detector device along a coaxial wave guide 82, the inner conductor of which becomes integral with anode 72 at one end thereof. Electromagnetic wave energy admitted to the detector along the coaxial wave guide 82 is propagated along the anode 72 and dissipated along the length thereof. Any residual electromagnetic wave energy may be absorbed by a lossy, resistive sleeve 84 which may be provided at the end thereof and which is interposed between the external terminal 76 and the body proper of the anode 72. The interior of the envelope 68 is filled with an ionizable gas at low pressure whereby consider-v able interaction between the electromagnetic wave energy and the ionizable gas takes place in the vicinity of the anode 72. External potential and load circuit connections may be the same as in the embodiment of invention shown in Fig. 3.

In the embodiment of my invention shown in Fig. 6 as in the embodiments of the invention previously described, a differential potential is developed across the terminals of the anodes 76 and 78, indicative of the presence or absence of a microwave propagated along coaxial wave guide 82 or indicative of the magnitude thereof. This potential differential is established by the increased interaction between the electromagnetic wave and the ionizable gas within the.envelope 68 in the region closely about the anode 72 over the same interaction taken place about the anode electrode 74 since the incoming microwave signal is directly applied ot the helical anode from the coaxial wave guide 82.

In accordance with another embodiment of my invention as shown in Fig. 7 of the drawings, a detector apparatus represented generally at 86 includes an enclosure 88 mounted on a wall of wave guide 90, containing a cylindrical cathode 92 in contact with leaf spring 94 mounted on the enclosure 88. For external circuit connections a lead 96 is connected to the spring 94. The cathode is preferably a circular, closed body and the space within the cathode 92 is filled with an ionizable gas at low pressure.

In accordance with a feature of this embodiment of my invention a pair of probe type anodes 98 and 100 are projected into the cathode and are insulated therefrom and from the wave gulde proper. To this end a circular and centrally apertured insulator 102 is disposed between one end of the cathode and a circular bracket 104, having a bore through which the anode extends and an apertured insulator 106 is mounted in a tubular bracket 108 for accommodating and supporting another outer portion of the anode 98. Anode 100 extends into an opposite portion of the cathode through circular, centrally apertured insulator 112 having an axially offset portion 113 fitting in the wave guide wall. The anode extends across the wave guide and into a stub portion 114 extending from an opposite wall of the wave guide. Each of the insulators is preferably bonded to parts it engages to produce a firm structure and a ring 115 rising on the wave guide wall about insulator portion 113 provides support for this insulator near its outer periphery. Accordingly, each of the anodes is insulated from the cathode structure whereby a direct potential applied from a source 116 between the cathode as a negative terminal and the anodes as positive terminals through respective current limiting variable resistors 118 and 120, establishes a gaseous discharge within the cathode. Such a discharge is of the type known as hollow cathode discharge.

Electromagnetic wave energy is propagated along Wave guide 90 at a predetermined frequency and the distance from anode 100 and a plunger 121 is made a quarter wave length at this frequency by manipulation of the plunger producing standing waves of maximum intensity in the region of anode 100. The wave is intercepted by anode 100 and introduced into the region within the cathode. Accordingly, an interaction between the microwave energy and the ionized gas occurs more particularly near the region of the anode rather than the anode 98 where? by the differential potential established across the anodes is indicative of the presence or absence or the magnitude of the microwave.

In each of the described embodiments of invention the anode resistors such as 22 and 24, in Fig. 1, may be of a value approximately 10K, it being understood, however, that resistors having diiferent values may be utilized according to the specific requirements of the circuit. The ionizable gas may be one of the noble or other chemically inactive gases at a pressure in the vicinity of 1 mm. of mercury. The gas pressure may range from 1 mm. of mercury to the extent of ten or more times in order to permit establishment of the necessary negative glow discharge region within the microwave propagating structure appropriate to the microwave frequency employed.

In the embodiment of the invention shown in Fig. 7 as in the other embodiments of the invention described hereinabove, the random variations of the negative glow plasma space potential caused largely by the primary electrons existing in this discharge type are minimized by the cancellation thereof in the respective anodes whereby the same fail to appear in the output potential taken across the anodes. Such primary electrons are of high energy since they are accelerated from a point at or near the cathode through the high field, cathode fall region into the region of negative glow.

While the present invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An apparatus comprising a hollow wave guide having a portion thereof sealed from ambient space, said portion having an ionizable gas at low pressure therein, a cathode extending along a length of said portion and a pair of anodes spaced from said cathode and being disposed along the length of said wave guide portion, means establishing a short circuit for said wave guide portion at a location adjacent to one of said anodes and means applying a direct potential on said anodes positive with respect to said cathode for ionizing said gas, where by modulated electromagnetic waves having a wave length substantially four times the spacing between said anode electrodes and propagated in said wave guide are detected to establish potential differential across said anodes indicative of said modulation.

2. An apparatus comprising a wave guide having an ionizable gas at low pressure therein, a cathode extending along a portion of said wave guide and a pair of anodes spaced from each other and from said cathode along said portion, means for applying a positive direct potential on said anodes with respect to said cathode for ionizing said gas and means for establishing standing waves having a wave length substantially four times the spacing between said anodes and having a voltage maximum in the region of one of said anodes.

3. An apparatus comprising a cathode and a pair of spaced anodes disposed in a sealed enclosure, an ionizable gas at low pressure within said enclosure and means for applying a positive direct potential on said anodes with respect to said cathode for ionizing said gas and means for establishing an electromagnetic wave field within said enclosure and having an intensity greater in the region of one of said anodes than in the region of the other of said anodes.

4. An apparatus comprising a sealed enclosure having a cathode electrode therein, a pair of anode electrodes spaced from said cathode electrode and spaced from each other, one of said anode electrodes having a surface area greater than the other, means for applying a positive potential on said anodes with respect to said cathode sufficient to establish ionization of saidgas within said enclosure and means for app-lying an electromagnetic wave to said anode having a larger surface area and means for observing the potential difierential between said anodes.

5. An apparatus comprising a sealed enclosure having a hollow cylindrical cathode therein and an anode extending within said enclosure along the interior of said cathode, another anode having a helical contour and extending along a portion of the interior of said cathode and means for applying an electromagnetic wave on said helical anode, means for applying a direct potential on said anodes positive with respect to said cathode to ionize said gas whereby a greater interaction between said fields and the ionized gas occurs in the vicinity of said helical anode than in the vicinity of said other electrode.

6. An apparatus comprising a sealed enclosure, a hollow cylindrical cathode within said enclosure and a pair of anode electrodes extending within said cathode, means for applying an electromagnetic wave to one of said anodes as a probe and means for applying a positive direct potential to said anode electrodes with respect to said cathode, said cathode electrode being filled with an ionizable gas at low pressure whereby an interaction between said electromagnetic wave introduced into said cathode by said one anode and said ionizable gas is greater in the region thereof than in the region of said other anode.

7. An apparatus comprising a hollow cylindrical cathode having an ionizable gas therein, a wave guide for propagating an electromagnetic wave of varying magnitude having an anode extending thereacross as a probe and extending within said hollow cathode through one end thereof, a second anode extending within said hollow cathode from the other end thereof and means for applying a positive direct potential to said anodes with respect to said cathode to ionize said gas and means for observing the potential differential between said anode electrodes in response to the variations in magnitude of said electromagnetic wave.

8. An apparatus comprising an enclosure containing an ionizable gas at low pressure, a pair of anode electrodes extending within said enclosure and means for applying a positive direct potential to said anode electrodes with respect to said enclosure, means for applying to one of said anodes a high frequency electromagnetic wave potential greater than at the other of said anodes whereby an interaction between said electromagnetic wave introduced into said enclosure and said ionizable gas is greater in the region of one of said anodes than in the region of said other anode.

References Cited in the file of this patent UNITED STATES PATENTS 1,978,021 Hollmann Oct. 23, 1934 2,409,991 Strobel Oct. 22, 1946 2,472,038 Yando May 31, 1949 2,631,255 Stavro- Mar. 10, 1953 

